How birth control, pregnancy, and abortion each impact a woman’s breast cancer risk

Christina Bennett 

Birth control, abortion, and breast cancer risk – Natural Womanhood

Given that one in eight women receive a devastating breast cancer diagnosis in their lifetime, virtually all women—myself included—want to know their personal risk factors for developing breast cancer and how to reduce their risk.  You may have heard that one common personal risk factor, hormonal birth control  use, increases your risk for breast cancer only slightlyaround 20%—and that this effect eventually goes away. A closer look at the evidence, however, shows the risk may be much higher for a particular group of women: namely, young women who use hormonal birth control before their first full-term pregnancy. They are at increased risk for developing breast cancer because their breast tissue is not fully developed, and is thereby especially susceptible to carcinogens (substances that can cause cancer) like the synthetic estrogen and progestin in hormonal birth control.  

Women Breast Cancer Support Charity Concept

Your breast cancer risk may be relative to childbearing and hormonal birth control use

In a recent Natural Womanhood article, “Does birth control prevent—or cause—cancer?” I discussed the increased breast cancer risk experienced by women on hormonal birth control, and the theory that birth control may increase breast cancer risk by overstimulating a woman’s breast cells. But research suggests that a woman’s breast cancer risk may also be related to when she begins using hormonal birth control. 

A 2007 meta-analysis of 34 studies showed that the risk of developing breast cancer before menopause was 19% higher for women who used birth control—which aligns with the often cited 20% increased risk for breast cancer [1]. But that risk rose to 44% when birth control was started before women had their first full-term pregnancy, and rose to 52% when birth control was used at least four years before their first full-term pregnancy.  

Young women who began birth control before age 20 had nearly double the risk of developing breast cancer by age 30 when compared with their peers who started birth control after age 20.

In a 2010 meta-analysis of 54 studies, young women who began birth control before age 20 had nearly double the risk of developing breast cancer by age 30 when compared with their peers who started birth control after age 20 [2]. That risk decreased as the group of women aged, but suggests that starting birth control at such a young age, when most women have not yet had children, could greatly affect breast cancer risk.  

Pregnancy and breast cancer risk

Angela Lanfranchi, MD, co-founder of the Breast Cancer Prevention Institute and Clinical Assistant Professor of Surgery at Robert Wood Johnson Medical School, has written an extensive peer-reviewed article as well as brochures about how breast tissue develops before and during pregnancy, and much of what follows is based on her writings and insights shared during an interview with Natural Womanhood.  

Birth control-induced changes to breast tissue make-up and why they matter

As Dr. Lanfranchi explains, the breast is made up of milk-producing structures called lobules, of which there are four types: 1, 2, 3, and 4. Each type represents a different stage of development, with type 1 being the least mature and type 4 being the most mature. Women who have never been pregnant and carried a child to term have breasts that are primarily made up of immature type 1 and 2 lobules. This makes their breasts more susceptible to carcinogens, because immature lobules have far more estrogen and progesterone receptors present than more mature type 3 and 4 lobules.

When type 1 or 2 lobules are exposed to estrogen and progesterone (including the synthetic versions found in hormonal birth control), their cells begin multiplying—and each time a cell splits into two, it runs the risk of making an error in the DNA, also known as a mutation. Mutations can lead to cancer.  “That’s why breast cancer will only start in immature lobules,” said Dr. Lanfranchi. She added that cancer simply “doesn’t happen” in type 3 or 4 lobules.  

Breast tissue make-up in women who have carried full-term pregnancies

In contrast, the breast cancer risk for women who have already carried a child to term may be less affected by hormonal birth control, precisely because their breast tissue has fully matured to type 4 lobules during pregnancy. 

By week 32 of pregnancy, or midway through the third trimester, 70% of the lobules have become type 4, and that proportion rises to 85% by the end of the third trimester. At delivery, the breast is made up of mostly mature, type 4 lobules—and, for the most part, once breast tissue has matured, it stays that way. And while the appearance of the breast lobules does transition to type 3 after a child stops breastfeeding, the protective benefits of a full-term pregnancy against breast cancer development are largely permanent.  

How breast tissue make-up is connected to the type of breast cancer diagnosis

Breast cancer that forms in type 1 lobules, known as ductal cancer, makes up about 85% of breast cancer diagnoses (type 1 lobules are mostly made up of ductal cells, hence why cancer in this area is named ductal cancer). Nearly the rest of breast cancer is found in type 2 lobules, and termed lobular cancer. Only rarely does breast cancer form elsewhere in the breast, such as in the muscle, fat, or blood vessels.  

When pregnancy ends prematurely, your breast cancer risk goes up 

Another important aspect of breast development is what happens to a woman’s breast tissue when she gets pregnant, but the pregnancy ends before full-term. Evidence suggests that when pregnancy is interrupted, either through an induced abortion, spontaneous miscarriage, or delivery before 32 weeks, the risk for breast cancer temporarily rises, because the types 1 and 2 lobules have multiplied but not yet matured into types 3 and then 4 lobules [3]. In essence, Dr. Lanfranchi explained, “Abortion [and, likewise, miscarriage and preterm delivery] increases your risk of breast cancer because it leaves your breasts with more places for cancers to start.”

To complicate matters, abortion can also increase breast cancer risk by hindering future full-term pregnancies. Abortion procedures can permanently weaken the cervix, a structure critical for carrying a baby to term. Research shows that pregnant women who previously had an abortion go on to have a higher risk of delivering their subsequent baby or babies early [4]. As with any woman who delivers prematurely, if a woman who has had a prior abortion goes on to have a subsequent pregnancy where she delivers before her type 1 and type 2 lobules mature around 32 weeks, she may not develop fully mature, cancer-resistant breast tissue–and therefore have a heightened risk for developing breast cancer. 

Given that 7 in 100 women taking hormonal birth control become pregnant within a year, and that many choose abortion with more than 600,000 procedures done in the United States in 2019 alone, women need better options for preventing pregnancy that do not involve synthetic hormones—and they need to be fully informed about the risks to their health that accompany abortion. Natural Womanhood encourages women to use fertility awareness methods, which have efficacy rates of preventing pregnancy that are equal or superior to the Pill, without the breast cancer risk and other harmful side effects of hormonal birth control.  


[1] Kahlenborn C, et al. Oral contraceptive use as a risk factor for premenopausal breast cancer: a meta-analysis. Mayo Clin Proc. 2006 Oct;81(10):1290-302. doi: 10.4065/81.10.1290. PMID: 17036554.

[2] Bjelic-Radisic, V., Petru, E. Hormonelle Kontrazeption und Brustkrebsrisiko. Wien Med Wochenschr 160, 483–486 (2010).

[3] Melbye, M., Wohlfahrt, J., Andersen, AM. et al. Preterm delivery and risk of breast cancer. Br J Cancer 80, 609–613 (1999).

[4] Shah PS, Zao J; Knowledge Synthesis Group of Determinants of preterm/LBW births. Induced termination of pregnancy and low birth weight and preterm birth: a systematic review and meta-analyses. BJOG. 2009 Oct;116(11):1425-42. doi: 10.1111/j.1471-0528.2009.02278.x. PMID: 19769749.

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